A modern energy-efficient light source module emitting white light is often based on a light emitting diode, LED, providing light in the blue spectral range and a wavelength converting element arranged to convert at least a part of the blue light into a green-yellow spectral range. The color point of the white light provided by such a light source module is determined by the ratio of the two spectral contributions, i.e. the blue and the yellow-green light. The color point of the light source module may be set to a given value by, for instance, the power of the blue light, the thickness, material and/or the structure of the wavelength converting element. Hence, it is possible to tailor the amount of light that is converted by the interaction with the wavelength converting element. Alternatively, additional optical components such as back-reflecting mirrors may be used to change the ratio of the two spectral contributions, i.e. the blue and the yellow-green light. To control the color point of the light emitted from the light source module is, however, often challenging as the conversion efficiency of the wavelength converting element is influenced by operating conditions, mostly important being the driving current and ambient temperature.
The light source module may have a separate beam of blue light that is additional to the beam of yellow-green light emitted from the wavelength converting material. The combining of light from the two beams allows for tuning of the color point of the white light emitted from the light source module. Although this approach is universal, it introduces extra complexity in the architecture of light source module and makes the optical design more complex.